Fix completion for cvs log command

Tim Connors reported the bug in Debian [1] and the fix has been
implemented upstream [2].  However, a new version of bash-completion has
not yet been released, so Debian carries the fix.

Thanks to Tim Connors for reporting.

[1] https://bugs.debian.org/892080

[2] https://github.com/scop/bash-completion/commits/b0851c06d050